1. Field of the Invention
The present invention relates to a sputtering magnetron having an adjustable shunt for fine tuning the magnetic field.
2. Description of Related Art
Magnetron sputtering is widely used for depositing layers of thin films of material onto a substrate. The process involves vaporizing a material by ion bombardment of a target which forms part of a cathode in an evacuated chamber containing an inert gas such as argon. A high voltage electric field is applied between the cathode and an anode in the chamber, and the gas is ionized by collision with electrons ejected from the surface of the cathode. The positive gas ions are attracted to the cathode surface and atoms of material dislodge when the ions strike the target, traverse the enclosure and deposit as a thin film onto a substrate positioned on a support maintained at or near anode potential.
In such a magnetron sputter coating process, the sputtering of materials from the sputtering target occurs most rapidly in regions of the target where the plasma trapped by the magnetic field is most dense. This localized region of trapped plasma erodes a narrow groove in the target, resulting in poor target utilization and generating a non-uniform distribution of sputtered atoms. Typically, the deposition of material from the target onto the substrate is correspondingly non-uniform, where often a film having a greater thickness at the target outer edge is sputtered as a function of target life.
Although the sputtering process can be carried out solely in an electric field, substantially increased and more uniform deposition rates are possible with magnetron sputtering in which an arched closed-loop magnetic field, formed in a closed loop over the surface of the sputtering target, is superimposed on the electric field. The arched closed-loop magnetic field traps electrons in an annular region adjacent to the surface of the target, thereby multiplying the collisions between electrons and gas atoms to produce a corresponding increase in the number of and better control of ions in that region.
Additionally, many applications of sputtering thin films require very precise uniformity tuning of the deposited layer. This is especially true for thin films which are individual layers of an optical interference filter such as low-emissivity films. New technologies for these films can require each layer to be within ±1.5% physical thickness across the entire substrate in order to meet product specifications.
Large magnetrons used for producing thin films have histories to companies that are no longer in existence. Many advancements have been made over the core technology which includes altering magnetic field so that the thin film uniformity can be tuned. It has been shown that changes as small as 1 mm, from a strategically placed shunt, can lead to uniformity shifts in the same location by an approximately 1%.
The sputtering magnetrons which are used to deposit these films can be tuned by altering the magnetic field profile/intensity within the magnet pack. Such tuning is required because, over time, the target erodes, the anodes become coated, gas flow shifts and as a result, the uniformity may drift out of specification. To re-tune the magnetron, the user must stop the process, physically remove the sputtering magnetron, and disassemble the cathode to make adjustments to the internal components, which are the magnets and the shunts. This procedure can cause a twelve hour or longer delay in production and then can require even further stoppage in the event more tuning is required. Furthermore, such tuning is required when a new target is installed.